I can tell ya how, but ya may not like the way I do it lol
The fastest way (for me) is to create a new temp thread with a poll. Then you need to make 2 quick changes to the Database to move the poll from the temp thread to the thread you want the poll in. You need the thread_id for the thread you want the poll in.
TABLE: xf_poll -- FIELD: content_id -- CHANGE ID from the Temp Thread to the Thread ID that you want to move the poll into
Then you need to edit the thread table to add a value to the discussion type field. (the row for the existing thread)
TABLE: xf_thread -- FIELD: discussion_type -- ADD THIS VALUE: poll